This invention relates to the preparation of crystals of wide band gap semiconductors for use in electronic devices.
As is known, crystals of semiconductors are of great interest in the manufacture of sophisticated electronic devices. The useful properties of semiconductive crystals depend not only on the particular semiconductor that forms the crystal but also importantly on the trace amounts of donor or acceptor dopants that are incorporated in the crystal lattice. These dopants contribute the hole and electron charge carriers that are responsible for the useful electronic properties of the crystals.
With many potentially useful semiconductive crystals, particularly those in which the host material is a wide band-gap semiconductor (a band gap of at least 1.4 electron volts), such as zinc selenide, it has been difficult to incorporate into the crystal lattice in reproducible fashion adequate amounts of both types of dopant to provide good p-n junctions, i.e., both n-type and p-type conductivity. As a consequence, it has not been practical to use such semiconductors widely in devices where bipolar conductivity is important.
In particular, although n-type zinc selenide has been available which conducts "well" (defined as having a resistivity equal to or less than 10.sup.3 ohm-centimeter), this has not been the case with p-type zinc selenide. The standard practice for preparing "well-conducting" n-type zinc selenide involves the extraction of impurities, such as copper, by annealing. Such annealing is known to extract copper that is accidentally introduced during the preparation of bulk material.
More recently n-type zinc selenide has been prepared using low temperature growth methods, such as metal-organo-chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) with selective incorporation of appropriate dopants.
However, with respect to p-type zinc selenide, it has been difficult to obtain "well-conducting" material reliably in reproducible fashion. There have been recent claims that such material has been realized by the introduction of Li.sub.3 N. Others have claimed to have obtained it by the introduction of gallium, indium or thallium by heat treatment and diffusion. However, the understanding of these techniques has been poor with the result that there is little confidence in applying these techniques to commercial products.
There is needed a better-understood technique, amenable to good control, for doping hard-to-dope wide band gap semiconductors, such as zinc selenide, zinc sulphide, cadmium sulphide, cadmium selenide, zinc telluride and diamond.